This invention relates to a method of producing 3-dibutylamino-6-methyl-7-anilinofluoran having a melting point in the range of 179.degree.-186.degree. C.
The contact of a colorless or only slightly colored proton donating leuco compound or a color former with an electron accepting acidic compound or a color developer under physical force or heat produces a colored substance. This is a well known phenomenon and is widely used in pressure or heat sensitive recording.
3-Dibutylamino-6-methyl-7-anilinofluoran is one of the color formers in wide use. It is known that the fluoran compound has three crystal modifications: high melting point compound which is characterized by peaks at diffraction angle (2.theta.) of 6.9.degree., 11.0.degree., 18.5.degree. and 18.9.degree. in X-ray diffractiometry using Cu-K .alpha. ray and having a melting point in the range of 179.degree.-186.degree. C.; a low melting point compound which has a melting point in the range of 146.degree.-155.degree. C.; and an amorphous type compound which has a melting point of 85.degree.-95.degree. C. Among these three crystal modifications, the high melting point compound is more excellent than any other in, in particular, color darkness and resistance to background coloration when used as a color former in pressure or heat sensitive recording material.
There is described in Japanese Patent Laid-open No. 60-202155 that a known conventional method provides a low melting type 3-dibutylamino-6-methyl-7-anilinofluoran. The conventional method is as follows. 2-(4-Dibutylamino-2-hydroxybenzoyl)benzoic acid and 4-methoxy-2-methyldiphenylamine are dissolved in concentrated sulfuric acid, and the reaction is carried out at 0.degree.-5.degree. C. for 20 hours. After the reaction, the reaction mixture is poured into ice water, whereupon precipitates are formed. The precipitates are collected by filtration and washed with water to provide a cake of 3-(4-dibutylamino-2-hydroxyphenyl)-3-(5-anilino-2-methoxyphenyl)phthalide. The cake is added to a mixture of toluene and water, and then the mixture is neutralized with a 28% by weight aqueous solution of sodium hydroxide. A further amount of sodium hydroxide is added and then the mixture is stirred at a refluxing temperature for three hours. After the reaction, the mixture is left standing, and a toluene layer is separated and concentrated at 70.degree. C. until a syrup is obtained. Methanol is added to the syrup so that 3-dibutylamino-6-methyl-7-anilinofluoran crystalizes out. According to the reference, the obtained compound has a melting point of 148.degree.-152.degree. C.
Thus, the above prior art reference proposes a method of producing the high melting point compound by first preparing the low melting point compound in the manner as described above, and then heating the compound at temperatures of not less than 70.degree. C. together with chlorobenzenes over a period of not less than 30 minutes.
A further method is also proposed in the reference, in which the phthalide is collected by filtration, added to a mixture of chlorobenzene and water, neutralized with an aqueous solution of sodium hydroxide, and then sodium hydroxide is added to the mixture, followed by stirring at 80.degree.-85.degree. C. Then a chlorobenzene layer is separated and heated at 110.degree.-120.degree. C. to convert the low melting point compound to the high melting compound.
The above methods need the treatment of the low melting compound with chlorobenzenes to convert it to the high melting compound. Thus, the methods are disadvantageous from the standpoint of process economy, and also the yield of the high melting point compound is low as the method needs steps of operations.